1. Field of the Invention
The present invention relates to a demodulator and a signal detecting method thereof, and more particularly to a sound intermediate frequency (sound-IF) demodulator and a sound-IF detecting method thereof.
2. Description of Related Art
Generally, the television system for each country will specify a number of channels within the UHF or VHF frequency ranges. A channel actually consists of two signals. One is the picture signal related the picture information transmitted using amplitude modulation on one frequency, and the other is the sound signal related to the sound information transmitted with frequency modulation (FM) at a frequency at a fixed offset (typically 4.5, 5.5, 6 or 6.5 MHz) from the picture signal.
In application of analog TV, a tuner selects one of the channels and frequency-shifts the signals to a fixed intermediate frequency (IF). Next, the IF signal is demodulated to a composite video broadcast signal (CVBS) and a sound signal by an IF demodulator. The FM sound signal is then demodulated and amplified by a sound-IF demodulator and used to drive a loudspeaker. Accordingly, if the sound-IF demodulator is exactly set to be matched with environment of source, the received signal can be demodulated perfectly.
However, the setting of the sound-IF demodulator may sometimes be mismatched with source signal in the real-world since the environment of source is changed. For example, for the TV program of china, the frequency deviation of modulated signal in commercial may be twice than the usual program. In this case, the sound-IF demodulator is set for usual program, and users endure the effect due to the wrong setting in commercial time, or the sound-IF demodulator is given a conservative setting, and users sacrifice the other program in the related art.
Furthermore, the environment of source may change among FM-mono, A2, and NICAM. Both of B/G signal and D/K signal are able to be transmitted through one of the above three methods. The environment of source may change after the sound-IF demodulator is set, and the sound-IF demodulator may be given a wrong setting for the environment of source.